Or course I remember Darmok. It's the second most famous Sinnlos im Weltall episode where Picard wants to buy a spaceship and ends up fighting an invisible monster with a Rambo knife. Don't pretend like that isn't why you remembered it as well.

That's the aforementioned "The missing link" (Das fehlende Fragment) where the missing link is the required data to have the replicator craft 50:50 oil:gas blend to make the moped run again. Easily the most famous episode, thanks to the talk about the virtues of black coffee.